1. Field of the Invention
The present invention relates to an electrically programmable read only memory (EPROM) and other semiconductor memory devices, in particular to an improvement of the light shielding structure of a cell transistor comprising a ROM for storing fixed information, e.g., a ROM for storing malfunctional addresses, provided for such semiconductor memory devices.
2. Description of the Related Art
In general, in an EPROM or other semiconductor memory device, in the program mode, predetermined data is programmed from a data programming circuit to a memory cell by storing, or not storing, electric charges in the floating gate of the cell transistor constituting the memory cell. On the other hand, in the read mode, the data programmed in the cell transistor is read out by sensing whether such electric charges are stored in the floating gate of the cell transistor constituting the selected memory cell.
To erase the data programmed in the cell transistors, i.e., to eliminate the charges stored in the floating gates of the cell transistors, ultraviolet light is irradiated over the entire chip on which the memory cell is formed from above the silicon insulating film formed on the silicon substrate.
Semiconductor memory devices are in general provided with a ROM for storing malfunctional addresses, for example, row addresses including malfunctional 30 memory cells, of the memory cell array. When an address signal, for example, row address signal, corresponding to a malfunctional address is input from outside, a redundant memory cell is selected and the memory cell corresponding to that malfunctional row address is not selected.
Recent ROM's for storing malfunctional addresses have been provided with floating gate type cell transistors of the same type as the cell transistors constituting the above-mentioned EPROM memory cells. Each bit constituting a malfunctional address is stored as fixed information by storing, or not storing, electric charges in the floating gate of the respective cell transistor.
When erasing the above-mentioned programmed data, however, i.e., when irradiating ultraviolet light on the cell transistors constituting the EPROM, the ultraviolet light must be prevented from also erasing the data programmed in the cell transistors constituting the ROM for storing malfunctional addresses, i.e., the data indicating the malfunctional addresses and other fixed data.
Therefore, in the past, consideration has been given to covering the surface of the cell transistors constituting the ROM for storing malfunctional addresses, i.e., the surface of silicon insulating film, with a light shield layer made of, for example, aluminum, so as to shield the cell transistors of the ROM for storing malfunctional addresses from the ultraviolet light.
In the prior art construction, however, the ultraviolet light would penetrate into the space between the drain terminal and the light shield layer, i.e., source terminal, of the cell transistors of the ROM for storing malfunctional addresses and through the silicon insulating film, i.e., phosphor silicate glass film, to reach the cell transistor portions. Of course, if the light shield layer covered a sufficient distance, the ultraviolet light which did make its way in from that space would gradually attenuate before reaching the cell transistor portions, but to ensure sufficient attenuation effect, a distance of coverage of a large value of, for example, several hundred microns, was necessary. The need to make the light shield layer such a large area resulted in the cell transistors constituting the ROM for storing malfunctional addresses being extremely large in size.
This problem is not limited to ROM's for storing malfunctional addresses, but is a common one for all ROM's for storing fixed information using an EPROM cell construction. For example, even when providing an EPROM with a ROM for storing hysteresis of the production process, test data, information regarding the type or product number of the memory device itself, or other fixed information, if that ROM is constructed with double gate transistors provided with floating gates like the EPROM cells, a light shield layer would be necessary and the above problem would occur.